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Adequate ventilation
The two side walls have a screened/meshed off two or three inch gap at the top and I keep the two sky lights on the "just open a crack" latch setting. I assume there's enough of a cross breeze to help as I've had no issues with rust on tools. There is what looks like water damage on the doors but that is from splashback in rain as the deck outside it comes pretty much right up to the threshold. It's done nothing more than discolour the wood at this point though. I tend to treat the front face of the shed when I'm doing the annual deck oil though which helps with beading a lot of that rain splash off.
I've only got 25mm Celotex insulating my shed and combined with a couple of thermostatically controlled tubular greenhouse heaters, it's kept my chisels dry for the past 10+ years
Adequate ventilation is provided by my half-arsed external cladding installation...